As described in the above-referenced '451 application, among the various conventional antenna assemblies that have been proposed for airborne and spaceborne applications are those which employ an inflatable medium, that may be unfurled from its stowed configuration to realize a `stressed skin` type of reflective surface. In such configurations, non-limiting examples of which are described in U.S. Pat. Nos. 4,364,053 and 4,755,819, the inflatable structure serves as the reflective surface of the antenna; namely, once fully inflated, the material is intended to assume and retain the desired antenna geometry.
Unfortunately, using the inflatable structure per se as the antenna surface creates several problems. First, the accuracy of the geometry of the antenna depends upon how faithfully the shape of the inflatable medium matches the antenna geometry, and also how well the shape of the inflatable medium can be maintained. Should there be (and there can expected to be) a change in the shape of the inflatable membrane, such as due to a change (most notably a decrease) in inflation pressure over time, the corresponding change in the contour of the inflatable structure will necessarily change the intended antenna profile, thereby impairing the energy gathering and focussing properties of the antenna. Although this inflation pressure decrease problem can ostensibly be addressed by the use of an auxiliary supply of inflation gas, it does not circumvent other causes of inflatable membrane distortion, such as, but not limited to, temperature and aging of the material, and particularly the fundamental ability of the inflated membrane to accurately produce the geometry of the antenna reflector.
In accordance with the invention described in the above-referenced '451 application, this inflation dependency problem is obviated by means of a hybrid antenna architecture, that effectively isolates the geometry of the antenna's reflective surface from the contour of the inflatable support structure, while still using its support functionality to deploy the antenna. For this purpose, rather than make the reflective surface geometry of the antenna depend upon the ability to maintain a prescribed pressure, the inflated membrane is employed simply as a deployable `tensioning` attachment surface. The inflatable tensioning membrane may support the tensioning tie/cord arrangement and the adjoining antenna surface either interiorly or exteriorly of the inflatable membrane.
FIG. 1 (which, except for the reference numerals corresponds to FIG. 2 of the '451 application) is a cross-sectional view of an exterior support embodiment of this hybrid antenna architecture. The hybrid structure of FIG. 1 is taken through a plane that contains an axis of rotation AX. A generally parabolic reflective surface 10 of the antenna is made of a lightweight, reflective or electrically conductive and material, such as, but not limited to, gold-plated molybdenum wire or woven graphite fiber. This surface is also rotationally symmetric about the axis AX, passing though an antenna feed horn 12.
The reflective surface 10 is attached by a tensioned cord and tie arrangement 20 to the exterior surface 31 of a generally toroidal or hoop-shaped inflatable support structure 30, which is also rotationally symmetric about the axis AX. The inflatable support structure 30 for the tie and cord arrangement 20 is joined to a support base 40 (e.g., a spacecraft) by way of a rigid truss attachment structure 50, that is formed of plurality of relatively stiff stabilizer struts or rods 51, also rotationally symmetric about the axis AX.
The inflatable hoop 30 may comprise an inflatable laminate of multiple layers of sturdy flexible material, such as Mylar. For deployment, the hoop 30 may be inflated through a valve 32, which may be located at or adjacent to its attachment to the truss 50, or the hoop may contain a material that readily sublimes into a pressurizing gas, that fills the interior volume 33 of the hoop 30.
The mesh reflector surface 10 is attached to the inflatable support structure 30 by means of tensionable ties 21 and cords 22 at perimeter attachment points 25, 27, distributed around the exterior surface 31 of the inflated membrane 30. This distribution of ties and cords is rotationally symmetric around the axis AX and is preferably made of a lightweight, thermally stable material, having a low coefficient of thermal expansion, such as woven graphite fiber. The hoop 30 is preferably inflated to a pressure greater than necessary to place the attachment cord and tie arrangement 20 at a minimum tension at which the reflective surface 10 acquires its intended shape.
This hybrid support structure enables the antenna surface to be maintained in a prescribed geometrical shape, that is independent of variations in the inflation pressure and shape of the hoop. Namely, the antenna is deployed and its geometry fully defined once the inflatable hoop is inflated to at least the extent necessary to place the attachment ties and cords at their prescribed tensions. Preferably, the inflation pressure is above a minimum value that will accommodate pressure variations (drops) that do not allow the hoop to deform to such a degree that would relax or deform the antenna from its intended geometry.